PSAs based on non-thermoplastic hydrocarbon elastomers such as natural rubber, butyl rubber, synthetic polyisoprene, ethylene-propylene, polybutadiene, polyisobutylene, or random styrene-butadiene random copolymer rubber are well known in the art. The dominant means of processing such adhesives comprises masticating the elastomer on a two roll mill or in a Banbury type internal mixer, dissolving the elastomer and other adhesive components in a hydrocarbon solvent, coating the solution onto a backing, and drying the coated product to remove the solvent. This technology is discussed in Handbook of Pressure Sensitive Adhesive Technology, D. Satas (ed.), p. 268. Van Nostrand, N.Y., (1989). The solvent process has the disadvantages of being labor intensive, having low production rates, and emitting large amounts of potentially hazardous solvents to the atmosphere thereby requiring expensive equipment for solvent recovery and/or incineration. Moreover, such solvent based processes have become increasingly undesirable for use in making adhesive tapes because of increasing environmental and safety regulations throughout the world.
Hot melt extrusion of pressure sensitive adhesives employing non-thermoplastic hydrocarbon elastomers such as natural rubber has been shown. However, low molecular weight plasticizing aids such as processing oils, elastomer oligomers, waxes, or other materials defined and described as plasticizers in Dictionary of Rubber, K. F. Heinisch, pp. 359-361, John Wiley & Sons, New York, (1974), are used as major components in the adhesive formulations. These plasticizing aids ease processing but detract from the ability of the finished adhesive to sustain a load and are generally known in the art to degrade adhesive performance.
WO 94/11175 describes a solvent free, hot melt process for non-thermoplastic hydrocarbon elastomers that obviates the need to use plasticizers as major components of the adhesive formulation. The process employs a continuous compounding device and hot melt processing techniques. The continuous compounding device has a sequence of alternating conveying and processing zones. The elastomer is continuously conveyed from one zone to another by the device. The processing zones are capable of masticating the elastomer. They are also capable of mixing additives into the elastomer. The adhesive composition can be applied to a moving web directly from the compounding device so as to provide a continuous method for the manufacture of PSA tape.
Oftentimes it is desirable to crosslink the coated PSA by exposure to ionizing radiation, such as electron beam or ultraviolet radiation, to enhance the cohesive strength (i.e., shear strength) of the material. To this end, crosslinking promoters are added to the elastomer to achieve higher crosslinking densities at lower radiation dosages. It has been found, however, that these crosslinking promoters (typically of the maleimide type) react with freshly scissioned elastomer, causing the viscosity to escalate. This, in turn, makes continuous processing using the techniques of WO 94/11175 very difficult.
In addition to these processing difficulties, it has also been found that adhesive tapes prepared by these techniques lack sufficient resistance to low stress peel for some applications. Such tapes are not able to effectively dissipate stresses such as, for example, those caused by temperature fluctuations, dimensional changes in the backing, and the recovery forces of a backing applied under tension. As a result, they may lift from a substrate to which they are applied.
Several attempts have been made to remedy the viscosity escalation and adhesive performance problems described above using techniques known in the art. For example, free radical neutralizing amines and hindered-phenol compounds have been added to adhesive formulations. It has been found, however, that the addition of these free radical neutralizing amines or hindered-phenol compounds to the process described above fails to overcome the viscosity escalation problem, and the tape properties (especially resistance to low stress peel) are not improved.
Thermosettable phenolic resins have also been added to adhesive formulations. However, in the solventless process described above, thermosettable phenolic resins begin to cure and crosslink in the compounder at the temperatures necessary to process the mix, causing small, tightly gelled impurities to form in the melt, resulting in processing and coating problems and product defects.
Tackifying resins can also be added in large quantities to improve resistance to low stress peel. The addition of tackifying resins, however, also typically increases the fast rate peel adhesion beyond acceptable levels with corresponding adhesive transfer problems and does not overcome the viscosity escalation problem.